1 List of United States cities by population. ^ 2  Population projection is that of the entity known since 2016 as Ciudad de MÃ©xico and previously as the Distrito Federal. ^  Population estimate is that of the Toronto census division, which consists solely of the City of Toronto.
Lens (optics) For other uses, see Lens. A lens is a transmissive optical device that focuses or disperses a light beam by means of refraction. A simple lens consists of a single piece of transparent material, while a compound lens consists of several simple lenses (elements), usually arranged along a common axis.
The lens, like the lens of a camera, opens and shuts to allow light to reach the retina to focus an image. With age, the lens loses the function to refract light to bring images into focus. Disease can rob the lens of its function to work with light. Many conditions can be corrected with surgery.